The IS 8870-1978 standard defines the specifications for asbestos cement cable conduits and troughs employed for power and telecommunication cable installations. It details the required dimensions, raw materials, manufacturing quality, and mandates performance evaluations such as flexural strength, impact resistance, and water absorption. This code is intended for producers and users who require robust and reliable asbestos cement conduits and trough assemblies with specified couplings and fittings for both subterranean and above-ground cable laying.
Overview
The IS 8870-1978 standard defines the specifications for asbestos cement cable conduits and troughs employed for power and telecommunication cable installations. It details the required dimensions, raw materials, manufacturing quality, and mandates performance evaluations such as flexural strength, impact resistance, and water absorption. This code is intended for producers and users who require robust and reliable asbestos cement conduits and trough assemblies with specified couplings and fittings for both subterranean and above-ground cable laying.
Audience
Contents
Structure
This section outlines the scope of asbestos cement conduits, troughs, and associated fittings, detailing size, dimensional tolerances, and configurations. It includes tables specifying nominal diameters, internal diameters, lengths, wall thicknesses, and permissible tolerances for both conduits and bends (90° and 135°). Similarly, it covers trough dimensions with nominal sizes, lengths, wall thickness, and allowable variations in depth, width, and length. Dimensions for couplings, collars, and rubber ring hardness (40–50 Shore) are also detailed for ensuring proper jointing and sealing.
Details the material composition comprising a homogeneous blend of ordinary Portland cement, asbestos fibers, and water for conduits and troughs. Specifies the dimensional standards for conduits with plastic and conical couplings and asbestos cement collars, including rubber ring specifications. Cable trough sizes and their associated dimensions such as width, height, radius, and hole counts are provided to maintain uniformity in production.
Summarizes the acceptable dimensional ranges and tolerances for asbestos cement conduits, bends, and troughs. It includes nominal diameters, lengths, wall thickness with allowable deviations, and tolerances for depth, width, and length variations in troughs. These specifications ensure interchangeability and correct fit during installation.
Emphasizes compliance with dimensional tolerances and surface finish standards for conduits, troughs, and fittings. Requires testing for straightness, uniform thickness, flexural strength, and water absorption. Detailed dimensional tables for various components ensure consistency in production, while rubber ring hardness ensures effective sealing.
Describes testing procedures including straightness, thickness uniformity, flexural strength, and water absorption for troughs, and impact resistance for conduits. Specifies load rates, span lengths, and minimum breaking loads for flexural tests. Impact resistance tests involve dropping a weight on samples placed on a V-notch support to determine minimum puncture forces.
Outlines formulas and test setups for flexural strength of troughs and impact resistance of conduits. Provides minimum breaking loads for various nominal sizes of troughs and impact force requirements for conduits based on diameter and drop height. Additional tests for trough straightness, thickness uniformity, and water absorption are included to validate durability.
Defines sampling methods in accordance with IS 7639-1975, requiring inspection lots to contain uniform items. Manufacturers are expected to perform quality control tests proportional to production, maintain certified test records, and allow purchaser inspections. Tables for dimensions and tolerances support inspection activities.
Mandates clear and durable marking on each conduit and trough with the manufacturer's name or trademark, nominal size, and the IS number (IS 8870). Optional ISI certification marks indicate compliance. Associated dimension tables provide reference for verifying product conformity.
Provides the procedure for impact resistance testing where minimum 300 mm conduit samples are tested by dropping a weighted cylindrical extension onto the conduit placed on a 120° V-notch support. Specifies drop heights and impact forces based on nominal diameter, with the test ensuring conduits resist puncturing under mechanical impact.
Describes the procedure to assess flattening resistance by applying a 250 N load on two 75 mm ring samples of conduit at 50°C for 48 hours. Diameter measurements before and after loading determine flattening percentage, which must not exceed 1%. This test confirms the conduit’s capacity to maintain shape under sustained compressive loads.
Details the 3-point bending test for troughs using a 1.5 m free span between hardwood bearers with a loading rate of 4.5 kN/min. Specifies minimum breaking loads for various nominal sizes to ensure troughs possess sufficient flexural strength to withstand operational stresses.
Frequently Asked
IS 8870 specifies the nominal sizes, lengths, wall thicknesses, and permissible dimensional variations for asbestos cement conduits and troughs. For example, troughs generally have a nominal length of 1.75 m, a wall thickness of 12 mm, and tolerances of ±3 mm for depth and width and ±6 mm for length. Conduits have specified internal diameters ranging from 50 to 150 mm with wall thickness tolerances of ±1.5 mm and length tolerances of +50/-20 mm. These standards ensure uniformity and proper fit during installation.
According to IS 8870, asbestos cement conduits and troughs must be manufactured using a uniform mixture comprising ordinary Portland cement conforming to IS 269-1976, asbestos fibers, and water. Couplings and collars may be fabricated from asbestos cement or plastic materials to provide reliable mechanical strength and electrical insulation suitable for cable protection.
The flexural strength test for cable troughs follows a 3-point bending method described in Appendix C of IS 8870. The trough is supported on two hardwood bearers spaced 1.5 m apart, with load applied at a rate of 4.5 kN per minute until failure. Minimum breaking loads vary by trough size, e.g., 2.5 kN for 100×100 mm and up to 10 kN for 300×300 mm troughs. The trough must withstand these loads without failure to meet the standard.
IS 8870 mandates that asbestos cement conduits resist puncture under impact forces achieved by dropping a weighted cylindrical extension from specified heights. For instance, a 50 mm diameter conduit must withstand an impact of at least 20 N from a 250 mm drop height, while a 150 mm conduit must endure 45 N from a 525 mm height. These impact thresholds ensure mechanical durability during handling and installation.
Water absorption testing is performed according to IS 5913-1970, involving drying the specimen to constant weight, immersing it in water for a set duration (usually 24 hours), and then weighing after surface drying. The water absorption percentage is calculated by the increase in weight relative to the dry weight. The average water absorption for conduits and troughs should not exceed 28% of the dry mass, ensuring product durability against moisture exposure.
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